xkbcommon-keysyms: Add header protection

xkbcommon-keysyms: Add header protection

As there is currently no stable release of xkbcommon, other projects might
want to include a copy of the keysyms so they can be used even though
libxkbcommon may not be available on the machine. However, if xkbcommon.h
is still included, conflicts will occur. Hence, to avoid nasty hacks,
simply include a header protection in xkbcommon upstream.

[daniels: Added protection to Makefile.am's update-keysyms, as well as
          XKB_KEY_NoSymbol, and a comment noting that it shouldn't be
          updated directly.]

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>
Signed-off-by: Daniel Stone <daniel@fooishbar.org>